This apparatus relates generally to particle separation and more particularly to the separation of particulate material through the use of upwardly moving streams of air or other gas.
The classification of particulate material according to density and/or aerodynamic properties by passing the particulate mixture through zones of differing air velocity has been known and practiced for a number of years. Air classification systems have been used for removing rocks or other foreign matter from such commodities as wheat, tea, raisins, wood chips and the like. A primary separation of light from heavy materials is an exceedingly important first step in the handling of heterogeneous particulate material. Because of the increasing cost of energy and raw materials the efficiency of this first separation step may be critical in determining the overall cost efficiency of a materials handling system.
Recently, compliance with environmental restrictions has necessitated the recycling of municipal garbage and industrial waste which in many cases are collected without discrimination and contain a diverse mixture of heavy materials such as glass, metal and stones, and of lightweight materials such as paper, leaves and plastic. It is advantageous to separate lightweight from heavyweight materials since in most instances, the lightweight material is combustible and thus usable as a source of energy if separated from the heavier materials.
A variety of different apparatuses have been proposed to perform particle separation. The efficiency of these prior art separators has been limited by features which were heretofore considered necessary for a successful separation process. Some of these apparatuses include complex duct arrangements to create turbulences in the material-bearing gas stream and thereby to improve material separation. Such designs are expensive to construct. Also, because of the high turbulence they create, a relatively great amount of energy is invested in moving a gas column through the tortuous ducts.
In other devices a stream of air moves upward in an essentially uninterrupted, straight column. A plurality of outlets on one side of the column are provided for materials to fall through according to their density. If, however, materials of any density or aerodynamic property migrate to the outlets of such a device, they fall through the outlets. The efficiency of separation is low because particles of low density and low aerodynamic characteristics will be carried out through outlets provided for the collection of denser or more aerodynamic particles.
In still other apparatuses heterogeneous material is carried into a series of columns having upwardly moving gas in each column. Because each column in the series contains gas moving upward at a velocity lower than that of the preceeding column, only those particles having the desired density or aerodynamic properties can fall through to the base of each column. Although the accuracy of separation in such devices is good, the operating costs have been relatively high since they have included numerous fans to be driven and many zones of high turbulence where particulate material and/or gas must reverse direction.